The invention belongs to the field of agricultural chemistry and provides to the art a method for controlling aquatic weeds. Such weeds clog waterways, plug up waterhandling equipment, and are often aesthetically unacceptable. They are cumbersome for fisherman, swimmers, and watersports. The economic impact for control and management in general and on recreational areas in particular is estimated to be in the millions of dollars.
A typical representative for inventively controlled aquatic weeds is hydrilla that is known as a submersed, very prolific, mat forming species, which can dominate the aquatic system that it is present in. High densities of hydrilla interfere with various water uses.
A typical representative is Hydrilla verticillata. 
Therefore, the development of methods for controlling aquatic weeds, in particular hydrilla, is important.
Generally, aquatic weeds and herbicidal or biological methods for controlling them are known, for example from L. W. J. Anderson, Pest Manag. Sci. 59, pages 801-813 (online 2003) or M. D. Netherland et al., Outlooks on Pest Management (Pesticide Outlook), pages 100-104 or J. Gallagher and W. T. Haller, Rev. Weed Sci., 1990, 5, 115-192.
Herbicides that are used for controlling aquatic weeds should fulfil one or more of the following requirements.
The compounds must be effective and efficient.
They should not be harmful to other plants than the ones to be controlled, to animals and man.
They are preferably degradable within a reasonable timeframe and the degradation products are harmless as well.
It is desirable that the compositions comprising the compounds used to control aquatic weeds have a slow activity and, therefore, less oxygen-depleting for the water. On the other hand, it may also be desirable that the compositions have a high activity which allows to eliminate fast-growing aquatic weeds in a short timeframe.
Langeland et al., “Effficacy of Herbicide Active Ingredients Against Aquatic Weeds” September (2006), University of Florida, indicates that 2,4-D and Triclopyr are not recommended for control of hydrilla. Klingman et al., “Aquatic-Weed Control”, Weed Science Society of America, Champaign, Ill., US (1982), pages 383-402 also shows in Table 29-1 that 2,4-D does not provide for control of hydrilla. 
WO 2007/071730 discloses that chinolin herbicides, like quinclorac and quinmerac, provide growth suppression or control of submersed aquatic weeds in general and of hydrilla in particular. Application methods disclosed in this document aim at a uniform dilution of the active ingredient in the water body.
According to this document, the herbicides are applied to the water column in liquid or solid formulations. Even though the disclosed herbicides are very effective, there is still room for improvement regarding the control of specific aquatic weeds or specific conditions.